fix(web): note save was re-rendering the editor, causing content to duplicate

[SwathiMystery]

Bug

Every save of a daily or trade note appeared to duplicate the textarea content. Hitting save a second time duplicated again.

Root cause

• `POST /notes/{trade|day}/...` returned the full `note_block` partial, and `hx-swap="outerHTML"` replaced `#note-block` wholesale.
• EasyMDE's CodeMirror sat inside `#note-block`. When the block swapped, the old CodeMirror stayed attached to the detached textarea while a new one mounted on the fresh textarea.
• The `htmx:afterSwap` re-mount listener was registered inside `wire()` — so every mount added another listener. Every save then fired N listeners, each attempting to re-mount. Multiple stacked CodeMirror instances all flushed into the same hidden textarea → content visually duplicated.

Fix

• New partial `partials/note_meta.html`: a single `<span id="note-meta-stamp">` with the saved-at timestamp.
• Save endpoint now returns just that span (78 bytes per save vs ~1.5 KB of editor HTML).
• Form's `hx-target` narrowed to `#note-meta-stamp`; editor DOM untouched.
• `editor.js`: CodeMirror `blur` now calls `htmx.trigger(form, 'khata-save')` — autosave without re-rendering.
• Removed the `htmx:afterSwap` re-mount listener (nothing swaps the editor anymore).
• Tests updated to assert the stamp-only response, then verify persistence via follow-up GET.

Verification

POST /notes/day/2026-04-15  (body=\"Hello from a save test\")
→ 200  78B  <span id=\"note-meta-stamp\" class=\"subtle\">saved 2026-04-21 04:29:42 UTC</span>
POST /notes/day/2026-04-15  (body=\"Second save\")
→ 200  78B  <span id=\"note-meta-stamp\" class=\"subtle\">saved 2026-04-21 04:29:42 UTC</span>
GET  /day/2026-04-15
→ editor textarea contains \"Second save\" exactly once.

Checklist

• 46/46 tests green
• `uv run ruff check khata tests` clean
• `uv run ruff format khata tests` applied
• No schema change