Task #466: Clean notes page from email-like 4-tab bar

User asked to remove the prominent 4-tab bar (My Notes / Inbox / Sent /
Archive) from the notes page because it made the page feel like an email
client, and they rarely use the share-between-users feature.

Changes (artifacts/tx-os/src/pages/notes.tsx):
- Deleted the inline-flex TabButton row that held the four view tabs.
- Page now opens directly on the unified "active" feed.
- Added an overflow DropdownMenu (⋯ MoreVertical icon) on the end of the
  controls row containing three items: Inbox (with unread badge), Sent,
  Archived. Each item calls setView() with the same TabId values, so all
  downstream branching (data fetching, filtering, composer visibility,
  folder rail) is unchanged.
- Unread inbox badge now appears (a) as a small numeric dot on the
  overflow trigger when not currently in Inbox view, and (b) inside the
  dropdown next to the Inbox item — the existing
  `data-testid="notes-inbox-unread-badge"` is preserved.
- Added a small "← My Notes" back button (testid notes-back-to-active)
  that appears only when view !== "active" so the user can return after
  drilling into Inbox/Sent/Archived without a visible tab bar.
- Removed the now-unused TabButton component definition.

Tests (artifacts/tx-os/tests/notes-inbox.spec.mjs):
- Replaced the two notes-tab-received / notes-tab-sent clicks with the
  open-dropdown-then-select-item sequence using the new overflow testids.

Validation:
- pnpm tsc --noEmit passes for @workspace/tx-os.
- Architect review: no severe issues; layout, a11y, RTL, z-index, and
  state-reset behavior all noted as sound.
- The `test` workflow has pre-existing failures (executive-meetings PDF,
  notes-share, groups-crud) that are explicitly out of scope per the
  task description and tracked by other open follow-up tasks.

No backend, schema, API, or i18n JSON changes were needed: the file
uses inline t() defaults, and `notes.tabs.received/sent/archived` keys
are reused inside the dropdown.

artifacts/tx-os/tests/notes-inbox.spec.mjs

@@ -158,7 +158,8 @@ test("sender → recipient → reply flow updates Inbox + Sent tabs", async ({
   await loginViaUi(page, recipient.username);
   await page.goto("/notes");
 
-  await page.getByTestId("notes-tab-received").click();
+  await page.getByTestId("notes-overflow-menu-trigger").click();
+  await page.getByTestId("notes-overflow-received").click();
   const inboxCard = page.getByTestId(`received-note-card-${createdNote.id}`);
   await expect(inboxCard).toBeVisible();
   await expect(inboxCard).toContainText(noteTitle);
@@ -203,7 +204,8 @@ test("sender → recipient → reply flow updates Inbox + Sent tabs", async ({
   await logout(page);
   await loginViaUi(page, sender.username);
   await page.goto("/notes");
-  await page.getByTestId("notes-tab-sent").click();
+  await page.getByTestId("notes-overflow-menu-trigger").click();
+  await page.getByTestId("notes-overflow-sent").click();
 
   const sentCard = page.getByTestId(`sent-note-card-${createdNote.id}`);
   await expect(sentCard).toBeVisible();