feat(lanes): Add Lane repo/worktree mode toggle, manual branch, folder browse

- Repo mode adopts a directory as-is via /lanes/ensure (no worktree, no
  branch fields) - the right choice for a main repo you want stage
  detection on. Worktree mode (default) keeps the existing provisioning
  flow but now requires a manually-typed branch name instead of deriving
  one from the title.
- POST /lanes/worktree accepts an optional `branch`, validated via
  `git check-ref-format --branch`; omitting it preserves the CLI's
  existing auto-derived-branch behavior.
- New GET /lanes/browse lists a directory's immediate subdirectories,
  backing a small folder-browse modal on both path fields - browsers
  cannot expose an absolute path from a native picker, so this is
  server-backed instead, consistent with the tool's local-first model.

client/src/i18n/locales/en/lanes.json

@@ -19,16 +19,29 @@
   "add": "Add lane",
   "addLane": "Create a lane from a working directory",
   "addLaneBaseLabel": "Branch to fork from",
+  "addLaneBranchHint": "Must be a valid, not-yet-existing git branch name.",
+  "addLaneBranchLabel": "New branch name",
+  "addLaneBranchPlaceholder": "feat/my-feature",
   "addLaneNoBranches": "This repo has no commits yet — the worktree will start empty.",
   "addLaneNotARepo": "Not a git repository (or no read access) yet.",
   "addLaneProvisionFailed": "Worktree provisioning failed or timed out — the lane exists but auto-setup was skipped. Check the lane's git facts, then run agents/mcp setup manually.",
   "addLaneRepoHint": "An existing git repo. The dashboard creates a new worktree for the lane, not a folder you pick.",
+  "addLaneRepoHintAdopt": "An existing directory. The dashboard tracks it as-is — no worktree, no new branch.",
   "addLaneRepoLabel": "Source repository",
+  "addLaneRepoLabelAdopt": "Directory",
   "addLaneSetupAgents": "Agents",
   "addLaneSetupMcp": "MCP servers",
   "addLaneSetupProfile": "Profile",
   "addLaneSetupTitle": "Setup",
   "addLaneTitleLabel": "Title",
+  "mode.repo": "Repo",
+  "mode.repoHint": "Adopt this directory as a lane, as-is — no worktree, no new branch.",
+  "mode.worktree": "Worktree",
+  "mode.worktreeHint": "Provision a new git worktree + branch from a source repo.",
+  "browse.button": "Browse",
+  "browse.empty": "This folder is empty.",
+  "browse.select": "Select this folder",
+  "browse.title": "Browse for a folder",
   "worktrees.heading_one": "{{count}} worktree",
   "worktrees.heading_other": "{{count}} worktrees",
   "addLaneTitlePlaceholder": "Optional",