feat: rename service to galo, add halo branding and release build

- rename Go module, binary, systemd unit, PG_DATABASE default, frontend package and docs from shared-album to galo
- add theme-aware halo emblem and favicon (new ui-branding capability)
- add make build-release (versioned, stripped, static binary) and run the binary from the repo root

openspec/changes/add-halo-branding/tasks.md

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+# Tasks
+
+## 1. Brand assets
+
+- [x] 1.1 Create `frontend/static/favicon.svg` with the halo mark (sun disc + separated ring) and embedded `prefers-color-scheme` dark/light variants, and verify it opens as valid SVG and renders legibly at 16px
+- [x] 1.2 Create `frontend/static/favicon.png` (fallback) and `frontend/static/apple-touch-icon.png` rendered from the same emblem, and verify both files exist at their intended pixel sizes (32x32 and 180x180)
+- [x] 1.3 Update `frontend/src/app.html` to reference `favicon.svg` with the PNG as fallback, and verify `make frontend` copies both into `frontend/build/`
+
+## 2. UI emblem
+
+- [x] 2.1 Add an inline SVG emblem component (disc in `--primary`, ring in `--primary` at reduced opacity, monochrome `currentColor` fallback), and verify it renders at 16-24px with the ring and disc still distinct
+- [x] 2.2 Verify the emblem changes color with the theme by switching light/dark/neutral and confirming the disc and ring follow the active theme
+
+## 3. Placement
+
+- [x] 3.1 Show the emblem in the `AppShell` header next to the "Galo" wordmark and verify it appears in the main interface
+- [x] 3.2 Show the emblem in the public/login layout and on the share page header and verify it appears on both
+
+## 4. Accessibility
+
+- [x] 4.1 Verify every decorative emblem (next to the wordmark) is hidden from assistive technologies and any standalone emblem element has an accessible name
+
+## 5. Integration verification
+
+- [x] 5.1 Run `make build` and verify the served favicon responds as an SVG image (`curl -I` shows the SVG content type) and that no brand asset request returns 404
+- [x] 5.2 Validate the change and verify `openspec validate add-halo-branding` passes