feat: add reminders, auto-delete, cross-device sync, dark mode, and rediscovery spotlight

- Rename extension from Shelf to "Read it later"
- Add reminder settings: highlight and pin overdue unarchived items
- Add auto-delete with a hard safety rule requiring its period to exceed the reminder period
- Add cross-device sync via chrome.storage.sync (lightweight per-item metadata, no thumbnails)
- Add light/dark/system theme support
- Fix list-view Archive/Delete button alignment (card-perf wasn't filling the row)
- Add "Worth a second look" spotlight: proactively resurfaces old, forgotten items with automatic backoff so repeatedly-ignored items are shown less often, never permanently

README.md

@@ -1,4 +1,4 @@
-# Shelf — Read Later
+# Read it later
 
 A new-tab page that shows everything you've saved to read later, as a
 sortable grid or list. No account, no server — everything lives in your
@@ -26,12 +26,48 @@ Thumbnails are captured automatically when you save the *current* page
 (a screenshot of what's on screen). Links saved via right-click show a
 favicon instead, since the browser hasn't loaded that page yet.
 
+## Reminders and auto-delete
+
+The gear icon opens a settings panel (also home to the light/dark/system
+theme picker) with two optional timers, both off by default:
+
+- **Remind me after** — items older than this get pinned to the top of the
+  shelf and highlighted, so they don't get buried.
+- **Delete unarchived items after** — silently removes items you never
+  came back to. Archiving an item takes it out of consideration entirely.
+
+The delete period must be longer than the reminder period — enforced both
+in the settings panel (an inline error if you try to set it shorter) and as
+a hard rule in the deletion logic itself, so you're always warned before
+anything is removed. Auto-delete runs on an hourly background alarm (so it
+still works even if you never open the new tab page), and again whenever
+the shelf page is open; when the shelf page does the deleting, it shows a
+courtesy toast with an **Undo**.
+
+## Worth a second look
+
+Above the shelf, a small strip proactively surfaces 2–3 old, forgotten,
+unarchived items each time you open a new tab — on by default, toggleable
+in the settings panel. Unlike a notification, it doesn't nag: the more
+times an item gets shown without you opening/archiving/deleting it, the
+less often it's picked again (backing off from every few days out to every
+couple of months), so nothing gets shown forever, but nothing gets shown
+*so* rarely that it's effectively forgotten again either. Archiving or
+deleting an item removes it from the strip immediately with no replacement
+until the next refresh — no whack-a-mole.
+
+"Old enough to matter" uses the same period as the reminder setting above
+when reminders are on, or a 14-day default when they're off. It's fine (and
+expected) for the same item to show up both pinned/highlighted in the main
+shelf *and* in this strip at once — they're not trying to be non-overlapping,
+just two different nudges.
+
 ## Install — Chrome / Edge (unpacked)
 
 1. Go to `chrome://extensions` (or `edge://extensions`).
 2. Turn on **Developer mode** (top right).
 3. Click **Load unpacked** and select this folder.
-4. Open a new tab — you should see the Shelf page.
+4. Open a new tab — you should see the Read it later page.
 
 ## Install — Firefox (temporary, for testing)
 
@@ -47,13 +83,26 @@ favicon instead, since the browser hasn't loaded that page yet.
 > [Developer Edition / Nightly build](https://www.mozilla.org/firefox/developer/)
 > with `xpinstall.signatures.required` set to `false`.
 
+## Syncing across devices
+
+Read it later keeps full items (including screenshot thumbnails) in
+`chrome.storage.local`, and also mirrors a lightweight copy of each item
+(url, title, domain, favicon, dates, archived state, spotlight backoff
+counters — no images) to
+`chrome.storage.sync`, one entry per item so each stays well under the
+8KB-per-item sync limit. On another device signed into the same browser
+account, opening the shelf pulls those in automatically; thumbnails just
+aren't part of what syncs, so items saved elsewhere show a favicon instead
+until re-saved on that device. Archiving and deleting also sync. This is
+all best-effort — offline, not signed in, or over the (~100KB / 512-item)
+sync quota just means that device keeps working from local storage alone.
+
+Note: `chrome.storage.sync` needs a stable extension ID to work, which
+only applies to a permanently installed, signed add-on — not a Firefox
+"temporary add-on" loaded via `about:debugging` (see install notes above),
+which won't persist sync identity across restarts.
+
 ## Notes / things you might want to change
 
-- Storage uses `chrome.storage.local`, so saved links stay on the device
-  you saved them on. Swapping the `chrome.storage.local` calls for
-  `chrome.storage.sync` in `background.js` and `newtab.js` would sync
-  small link lists across your signed-in browsers, but `sync` storage
-  has a much smaller quota (~100KB) and doesn't handle thumbnails well —
-  not recommended once you have images in the mix.
 - Icons in `icons/` are placeholders — swap them for your own artwork
   any time; sizes 16/32/48/128 are already wired up in `manifest.json`.