feat(THI-244): default cwd setting + new-pane cwd inheritance (sessions mode)

[tdody]

Summary

Today the +session / +window overlays start panes wherever tmux decides (usually the server-process cwd), so users routinely retype the same paths. This PR ships the Sessions-mode half of THI-244: a configurable default cwd plus the "+window inherits its session's cwd" behavior.

Resolution order (matches the spec's sessions-mode rules):

1. Per-modal override (one-shot input on each overlay).
2. For +window: the launching session's first window cwd, looked up server-side. Matches the user's expectation that "+" lands next to peers.
3. For +session: the configured Default directory setting.
4. Otherwise None — tmux uses its own default.

What ships

Backend

• POST /window and POST /session accept an optional CwdBody ({cwd}).
• _resolve_cwd validates: ~-expansion against the server's $HOME, must be an existing absolute directory. Anything invalid silently resolves to None so a bad setting never breaks new-pane creation with a 4xx.
• _first_window_cwd looks up the launching session's first window cwd via libtmux; used as the fallback when the body has no explicit cwd.
• 8 new test cases cover the resolver + both routes' threading paths (explicit cwd, invalid cwd, relative cwd, ~ expansion, first-window fallback).

Frontend

• New defaultDirectory: string setting (empty default). Persisted to localStorage like every other setting.
• New Defaults section in SettingsModal with the text input.
• createSession / createWindow API helpers accept an optional cwd and body-encode it as JSON when provided.
• NewSessionOverlay prefills its cwd input from settings; the user can override per-modal without mutating the saved setting.
• NewWindowOverlay adds the same one-shot cwd input; blank means "let the backend fall back to this session's first window cwd".
• 4 new tests (2 for settings round-trip, 2 for client.ts cwd threading).

Explicitly deferred

The discovery-mode auto-derive half of the spec ("in discovery mode, ignore the global default; use the owning repo's path") depends on:

1. THI-243 landing.
2. THI-243's discovery UI exposing per-repo + buttons.

THI-243's MVP doesn't yet expose those (ListView only renders headers, no per-repo create affordance). The hook isn't reachable, so the discovery branch lands as a small additive change in a follow-up. This PR's resolution function already has the right shape — adding a (repoKey?: string) => string branch on top will be a one-liner.

Test plan (automated)

• Backend: 380 tests pass (uv run pytest). ruff check + ruff format --check: clean.
• Frontend: 553 tests pass (npm test). Typecheck + build: clean.
• New tests: 8 backend (resolver + 2 routes) + 2 client.ts + 2 settings = 12 new tests.

Manual smoke (needs a live tmux server — only the user can run this)

Settings UI

• Open Settings → scroll to Defaults. The "Default directory for new sessions" input is present.
• Enter ~/dev (or some path you have). Close + reopen Settings: the value persists.
• Enter a garbage path like /nope/nope/nope. Save it. (Backend will silently drop on use — no toast / error in Settings itself.)

New session

• Click "+ session". The cwd input is prefilled with the configured Default directory.
• Create the session, focus into it in tmux (tmux attach), run pwd. It should be the configured directory (or $HOME if you used ~).
• With the garbage path saved as default, create a new session. It should still succeed (no error), landing in tmux's fallback dir (the switchboard server-process cwd).
• In the modal, override the prefill with a different path. Confirm pwd matches the override, not the saved default.

New window

• Pick an existing session whose first window has a known cwd (e.g. ~/foo).
• Click "+ window" on that session — leave cwd blank.
• tmux attach, jump to the new window, pwd. It should match the launching session's first window cwd.
• Try "+ window" with an explicit cwd override. Confirm pwd matches the override.
• Try "+ window" with a non-existent cwd like /nope/nope. Confirm the window is created (no error) and lands in tmux's fallback.

Edge cases

• Set Default directory to ~ only. New session lands in $HOME (server-side expansion).
• Set it to a relative path like subdir. New session falls through to tmux's default (relative paths rejected server-side).

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