feat: autosave drafts, and stop losing them on a switch

Only Ctrl-S and :w ever wrote. A closed laptop, a crashed tab or an
expired session took the draft with it, and clicking another entry in the
list silently discarded unsaved work without so much as a prompt.

Every change now goes to localStorage immediately and to the server two
and a half seconds after typing stops. Opening an entry that has a stash
newer than the server's copy restores it and says so, the way vim reports
a swap file; :e! and :q! discard it. Switching entries flushes the
outgoing draft first.

The save response carries the entry's excerpt, so a save patches its list
row in place instead of re-reading the whole list — which autosave would
otherwise have done every few seconds.

web/src/lib/api.ts

@@ -12,6 +12,7 @@ export interface Entry {
   id: number
   title: string
   body: string
+  excerpt: string
   created_at: number
   updated_at: number
   shared: boolean