feat: invitation accept screen and api

features/accept_invite_screen.feature

@@ -0,0 +1,105 @@
+Feature: Accept invite screen
+
+  As a person who opened the link in the invitation email
+  I want /invite/accept?c=CODE to show me the invited address and let me choose a username and
+    password
+  So that I finish setting up my account and arrive logged in
+
+  Background:
+    Given /invite/accept is a public route (meta.auth "public") rendered as AcceptInvitePage inside
+      AuthShell, per docs/design/routes.md
+    And on mount AcceptInvitePage reads the "c" query parameter once: if it is absent or empty no
+      request is made and the "Invalid or used link (404)" state renders
+    And otherwise it calls GET /v1/auth/invite?c=CODE and shows "Checking the link" until it resolves
+    And the Username and Password rules are Register's, including PasswordStrengthHint, with no
+      confirm field; validation runs on blur and on submit and nothing is sent when it fails
+    And a submit that passes calls POST /v1/auth/invite/accept with { code: c, username, password }
+    And a logged-in visitor sees the same screen: the route stays public and nothing redirects
+    And the board is docs/design/source/RenewCMS Screens Auth.dc.html, group "Accept invite"; the
+      board frame decides layout and sample data (lea.martin@renew.example, "lea", "limewash2026",
+      "marta")
+
+  Scenario: Checking the link
+    When the user opens /invite/accept?c=abc123 and GET /v1/auth/invite has not resolved
+    Then a full-card spinner reads "Checking your invite…"
+
+  Scenario: Default
+    Given GET /v1/auth/invite?c=abc123 resolves 200 with {"email": "lea.martin@renew.example"}
+    Then the heading is "Accept your invite"
+    And the description is "Choose a username and password to finish setting up your account."
+    And the Email field has placeholder "you@example.com", shows "lea.martin@renew.example", is
+      read-only, with help "From your invitation."
+    And the Username field has placeholder "e.g. lea", counter "0 / 20" and help "Letters and numbers,
+      3 to 20 characters. You'll use it to log in."
+    And the Password field is a password input with show/hide and checklist "At least 8 characters" unmet
+    And the submit button reads "Accept invite"
+    And the footer reads "Already have an account?" with a "Log in" link to /login
+
+  Scenario: Validation errors
+    Given the form is shown, Username is "ma" and Password is "lime"
+    When the user blurs both fields and submits
+    Then POST /v1/auth/invite/accept is never called
+    And the Username shows counter "2 / 20" in error and the error "Use 3 to 20 characters."
+    And the checklist item "At least 8 characters" is in its error state
+
+  Scenario: Filled, valid
+    Given Username is "lea" and Password is "limewash2026" with the password shown
+    Then the counter reads "3 / 20", the checklist item "At least 8 characters" is met and no error shows
+
+  Scenario: Submitting
+    When the user submits valid values and POST /v1/auth/invite/accept has not resolved
+    Then the fields and button are disabled
+    And the submit button reads "Creating account…" and is loading
+
+  Scenario: Username taken (409)
+    When POST /v1/auth/invite/accept resolves 409 with body "That username is taken. Try another."
+    Then the Username keeps "marta", shows counter "5 / 20", is focused and shows the error
+      "That username is taken. Try another."
+    And the Password keeps its value, the form re-enables and the button reads "Accept invite"
+
+  Scenario Outline: The link does not work
+    When <trigger>
+    Then no form is shown
+    And a ResultPanel (tone error, icon i-lucide-link-2-off) reads "This invite link doesn't work"
+      with text "It may be incomplete, or it may already have been used. If you already set up your
+      account, log in instead." and note "Open the link from your email again, or copy the full
+      address into your browser."
+    And its only action is a primary "Go to log in" that navigates to /login
+
+    Examples:
+      | trigger                                                                |
+      | the user opens /invite/accept with no c parameter (no request is made) |
+      | GET /v1/auth/invite?c=abc123 resolves 404                              |
+      | the submit resolves 404, replacing the form                            |
+      | the submit resolves 409 with apperr.EmailHasAccountErrorMessage        |
+
+  Scenario Outline: Expired link (410)
+    When <trigger>
+    Then no form is shown
+    And a ResultPanel (tone error, icon i-lucide-clock) reads "This invite has expired" with text
+      "Invites work once, for 3 days. Ask the person who invited you to send a new one."
+    And its only action is a secondary "Go to log in" that navigates to /login
+
+    Examples:
+      | trigger                                     |
+      | GET /v1/auth/invite?c=abc123 resolves 410   |
+      | the submit resolves 410, replacing the form |
+
+  Scenario Outline: Server error
+    When the submit <failure>
+    Then an error Alert (icon i-lucide-circle-x) reads "Couldn't create the account" with "Something
+      went wrong on the server. Try again in a moment." above the heading
+    And the form is still shown with Email, Username "lea" and Password "limewash2026" kept, re-enabled
+
+    Examples:
+      | failure                                |
+      | resolves 500                           |
+      | resolves 429                           |
+      | never reaches the server               |
+
+  Scenario: Success
+    When POST /v1/auth/invite/accept resolves 200 with the jwt cookie
+    Then a ResultPanel (tone success, icon i-lucide-circle-check) reads "Your account is ready" with
+      text "Taking you to your dashboard…"
+    And a Toast (icon i-lucide-circle-check) reads "Welcome to RenewCMS" with "You're logged in as lea."
+    And after 800 ms the app navigates to /home with no verification step