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/commands.ts

@@ -107,7 +107,10 @@ export const commands: CommandSpec[] = [
         const open = diary.open
         diary.editing = false
         diary.dirty = false
-        if (open) await diary.guard(() => diary.openEntry(open.id))
+        if (open) {
+          diary.dropStash(open.id)
+          await diary.guard(() => diary.openEntry(open.id))
+        }
         hooks.focusList()
       },
     },
@@ -168,6 +171,7 @@ export const commands: CommandSpec[] = [
         const open = requireOpen()
         if (!open) return
         diary.dirty = false
+        diary.dropStash(open.id)
         await diary.guard(() => diary.openEntry(open.id, diary.editing))
         diary.say(`entry:${open.id} reloaded`)
       },