fix(dash0): stop pre-filling a webmail domain as the first check target

[fclairamb]

Three defects on the signup path, found by reading PostHog replays of real signups that ended with zero checks.

1. Fixed: the first check target was the user's mail provider

The zero-check dashboard pre-fills the signup's email domain as the URL to monitor, and skips free webmail. The skip list only held .com names. A recent @gmx.de signup got gmx.de pre-filled, never typed anything, and left without a check. Same for @yahoo.fr, @hotmail.co.uk, @web.de, @orange.fr and so on.

• Country variants of the big providers are matched by brand: gmx.de, gmx.net, yahoo.co.uk, hotmail.fr, mail.ru. A brand label deeper in the name is not (mail.acme.com is still suggested).
• National providers are listed: web.de, t-online.de, orange.fr, free.fr, laposte.net, libero.it, btinternet.com, uol.com.br...
• The same rule backs validateAutoJoinRegex, so an admin can no longer set .+@gmx\.de and auto-admit every signup from that provider. Nothing sets that pattern automatically, so this was defense in depth only.

Files: server/internal/handlers/auth/autojoin_regex.go (source of truth) and web/dash0/src/lib/quick-target-suggestion.ts (mirror), with tests on both sides.

2. Spec: a first-time visitor is told their session expired

specs/todos/2026-10-12-01-first-visit-no-session-expired-banner.md. Opening solidping.io in a fresh browser lands on /d/orgs/default/login?session_expired=true with "Your session has expired". Cause: RootRedirect sends a visitor with no session to /no-org, its requests 401, and handleResponse always calls redirectToExpiredLogin(), which always sets the flag.

3. Already filed: signing up from the demo files a join request to demo

Covered by specs/todos/2026-10-10-02-demo-org-no-membership-request.md, nothing added here.

Checks

• go test -short ./internal/handlers/auth/ passes
• golangci-lint run ./...: 0 issues
• vitest run src/lib/quick-target-suggestion.test.ts: 51 passed
• eslint clean on the changed files

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.76%. Comparing base (44840fd) to head (0e88181).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #532      +/-   ##
==========================================
+ Coverage   82.71%   82.76%   +0.04%     
==========================================
  Files        1175     1175              
  Lines      121451   121460       +9     
==========================================
+ Hits       100463   100525      +62     
+ Misses      20981    20928      -53     
  Partials        7        7              

Files with missing lines	Coverage Δ
server/internal/handlers/auth/autojoin_regex.go	97.77% <100.00%> (+0.55%)	⬆️

... and 22 files with indirect coverage changes

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