fix(gui): improve mobile controls, attachment delivery, and navigation

[bborn]

Improve the GUI's mobile controls, attachment delivery, and navigation based on phone and browser dogfooding.

• Put filters in the header, with Apply, immediate Reset, a clear-search button, resettable saved views, and persistent drafts/collapsed groups.
• Add a shared Board/List switch, styled grouping/sorting controls, project kanban columns, roomier list rows, and accurate keyboard help. Settings toggles closed from the header and exposes Appearance, Projects, Task types, Routines, Machines, and Executors. Machine discovery uses the existing placement integration; host editing is not added.
• Make the reply composer open the native picker directly, show persistent pending files, accept attachment-only replies and pasted images, and retain drafts when delivery fails. Remove canned yes/continue replies and move Retry into task actions.
• Store attachment bytes as local checksum-named files beside SQLite; stage execution copies in the task worktree on the selected host. SSH transfers verify checksums and fail before prompt delivery. Upload requests allow two minutes with a bounded body size.

Existing blobs migrate on database open. Schema 3/protocol 4 requires restarting older executors. Backups must include the attachment directory; migration does not vacuum or prune logs. See docs/attachments.md. The main user database was only inspected read-only.

Validation: frontend production build and kanban grouping checks pass; changed Go packages pass, covering migration/integrity, local and fake-SSH staging, failure handling, and attachment-only HTTP prompt delivery. Browser checks covered persisted filters/groups, Reset, direct upload and failure retention, layout switching, project columns, key-badge sizing, Settings navigation, routines/machine/executor listings, and viewport overflow checks documented in docs/qa/mobile-web-2026-09-16.md.

Limits: the full Go suite encountered TestRemoteAttachDropsALeftOverViewPairing (tmux window lookup, also reproduced alone). Installed golangci-lint targets Go 1.25 and cannot lint this Go 1.26 repository. Real remote-agent delivery and new iPhone software-keyboard checks remain unverified. No additional macOS Actions jobs were started.