fix(svg): pulse today's tile even when commit count is zero (JhaSourav07#1656)

## Description
Fixes JhaSourav07#1648 

Today's tile on the isometric grid had no visual indicator or pulse
animation when the user had zero commits for the day. This made it
impossible to locate "today" on the grid and removed any motivational
nudge to make a commit.

**Changes made:**
- Updated `lib/svg/layout.ts` to assign the `TODAY:` tooltip label to
today's tile even when contribution count is `0`
- Updated `lib/svg/generator.ts` (`renderTowers` and
`generateAutoThemeVersusSVG`) to trigger the breathing opacity pulse
animation on today's tile regardless of commit count
- Added a prominent accent stroke outline around today's flat top-face
diamond when commit count is `0`, using the accent color for a premium
highlighted look
- Added and updated unit tests in `lib/svg/layout.test.ts` and
`lib/svg/generator.test.ts`
- All 126 tests pass

## Pillar
- [ ] 🎨 Pillar 1 — New Theme Design
- [x] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [ ] 🛠️ Other (Bug fix, refactoring, docs)

## Checklist before requesting a review:
- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=Pranav-IIITM`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [ ] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07