fix: support project opening on headless hosts

[danyaberezun]

Summary

• distinguish native folder-picker cancellation from display and execution failures
• recover headless and remote project opening with an in-browser host-path dialog and an always-available manual action
• accept absolute host paths plus ~ while rejecting ambiguous relative paths

Closes #328

Changes

Host picker

• detect Linux hosts without DISPLAY or WAYLAND_DISPLAY before launching a graphical picker
• classify picker results from exit code and diagnostics instead of treating every non-zero Linux exit as cancellation
• fall through to another picker after execution failure while preserving genuine cancellation as { path: null }
• keep the existing wire shape and surface failures through the existing rejected-request path

Browser recovery

• add an Open project from host path dialog with the host failure reason, autofocus, and keyboard submission
• add Enter host path… to both Add Project entry points
• ignore late replies from superseded native pickers so an invisible host dialog cannot later switch projects
• reuse the existing open/inspect/init orchestration for manual paths

Host path handling

• resolve exact ~ and ~/… against the host account
• apply one path policy across project open, inspect, and initialization
• reject other relative paths instead of interpreting them against the host process cwd

Specs and coverage

• update the dialog, projects, panels, and e2e specs
• cover headless preflight, cancellation, diagnostic failure, picker fallback, path expansion, relative-path rejection, explicit manual opening, and automatic browser recovery

Testing

• bun run check:deps — passed
• bun run check:seams — passed
• bun run lint — passed
• bun run typecheck — 11/11 package tasks successful
• bun run test — 11/11 package tasks successful
• THINKRAIL_E2E_SHARDS=4 bun run e2e — 300 passed across 4 shards
• bun run e2e -- --last-failed — 2/2 unrelated timing failures from the initial 8-shard run passed in isolation before the complete 4-shard run

Screenshots

Add Project menu

Before	After

Headless picker result

Before	After

[jetbrains-air]

Approved — ready to merge.