fix(dashboard): show failed saves with a retry

[WhiteMuush]

Closes #37 (closed manually on merge, closing keywords do not apply to develop).

Bug

Every dashboard preset change ignored server failures: layout save, switching view, creating, renaming, deleting, changing a widget source, resetting. On a failed request the change looked saved and was lost on reload, or a deleted view came back.

Fix

• saveRequest (src/lib/save-request.ts) returns null on a refused request or a network error, so a failure cannot be ignored by accident. Unit tested.
• A failed save shows a bar: "Your last dashboard change was not saved", with Retry (replays the whole action) and a close button.
• Rename and delete only apply once the server accepts. Create already did.

DashboardCanvas.tsx was at its 776-line cap: the Ctrl+wheel zoom moves to useCtrlWheelZoom.ts unchanged, and the file drops to 734 lines, with its cap tightened to match.

Verified in the browser (requests forced to 500, then allowed)

• Create fails: bar shown, no new tab. Retry: tab created.
• Rename fails: name unchanged. Retry and reload: new name kept.
• Delete fails: tab stays. Retry and reload: tab gone.
• Ctrl+wheel zoom still works (scale 1 to 1.82). No page error.

make check green.