feat(todo): switch /todo view to todo.txt format

Replace the markdown _todo/todo.md view with a structured todo.txt UI at the
repo root: priority dropdown, inline body edit, project/context/due/rec chips
with Tabler icons, add input as a join-suffix, project/context filter columns
with TransitionGroup animation, priority-sorted list. Drops the now-unused
.todo-notes markdown checkbox styles from app.css.

src/hooks/useTodoTxtCommit.hook.ts

@@ -0,0 +1,87 @@
+import { useDebounceFn } from "@vueuse/core"
+import { onUnmounted, Ref, ref, toValue } from "vue"
+
+import { useGitHubContent } from "@/hooks/useGitHubContent.hook"
+import { FileLine, parseFile, serializeFile } from "@/utils/todotxt"
+
+export const useTodoTxtCommit = ({
+  user,
+  repo,
+  path,
+  initialContent,
+  initialSha,
+  debounceMs = 1000
+}: {
+  user: string
+  repo: string
+  path: Ref<string | undefined> | string | undefined
+  initialContent: Ref<string> | string
+  initialSha: Ref<string> | string
+  debounceMs?: number
+}) => {
+  const { updateFile } = useGitHubContent({ user, repo })
+
+  const items = ref<FileLine[]>(parseFile(toValue(initialContent)))
+  const currentSha = ref(toValue(initialSha))
+  const isCommitting = ref(false)
+  const hasPendingChanges = ref(false)
+
+  // Re-seed from a freshly fetched file. We must not blow away in-flight edits.
+  const syncContent = (content: string, sha: string) => {
+    if (!hasPendingChanges.value) {
+      items.value = parseFile(content)
+      currentSha.value = sha
+    }
+  }
+
+  const commitChanges = async () => {
+    const pathValue = toValue(path)
+    if (!pathValue || !hasPendingChanges.value) {
+      return
+    }
+
+    if (isCommitting.value) {
+      debouncedCommit()
+      return
+    }
+
+    isCommitting.value = true
+
+    const { sha: newSha } = await updateFile({
+      content: serializeFile(items.value),
+      path: pathValue,
+      sha: currentSha.value
+    })
+
+    if (newSha) {
+      currentSha.value = newSha
+      hasPendingChanges.value = false
+    }
+
+    isCommitting.value = false
+  }
+
+  const debouncedCommit = useDebounceFn(commitChanges, debounceMs)
+
+  const mutate = (mutator: (current: FileLine[]) => FileLine[]) => {
+    items.value = mutator(items.value)
+    hasPendingChanges.value = true
+    debouncedCommit()
+  }
+
+  onUnmounted(() => {
+    // Flush any pending edit on unmount so navigation doesn't lose work.
+    if (hasPendingChanges.value) {
+      commitChanges()
+    }
+  })
+
+  return {
+    items,
+    currentSha,
+    isCommitting,
+    hasPendingChanges,
+    syncContent,
+    mutate
+  }
+}