Web UI: Updated tab, inline errors, mobile card fixes (#3)

Third pass on the password-gated web UI, on top of #1 and #2.

## Updated tab
New `?tab=new` tab listing only series with an unread published chapter, plus per-tab empty states for Favourites and Updated. Covered by `TestUIListNewTab`.

## Inline error feedback
htmx does not swap on a non-2xx response, so a failed favourite/chapter/delete looked like an ignored tap. Errors now render in a `.error-inline` slot on the card and clear after 5s. The chapter-edit form and delete-confirm row also close each other — only one per-card panel open at a time.

## Mobile fixes (P0)
`.chapter-form` held three children on one unwrapped flex row, pushing Save off screen: **97px of page overflow at 390px, 127px at 360px**. That broke correcting a chapter number on the primary device class.

- `.chapter-form` and `.confirm-row` wrap; hint and prompt take their own full-width row
- `.chapter-form input` uses `flex: 1 1 0; min-width: 0` — with `flex: 1` (basis auto) a number input holds its ~20ch intrinsic width and refused to shrink, which pushed Save to a third row
- `white-space: nowrap` on the confirm prompt alone reintroduced 26px of overflow; the full-width row is what actually fixes it

Verified live: `document.body.scrollWidth <= window.innerWidth` with every chapter-form and confirm-row open, at 360/390/768/1280, light and dark.

## Icons
`☆ ✎ 🗑 ▶` replaced with hand-authored inline SVG on `currentColor` — the emoji font rendered each in a different face, weight, and colour, ignoring the card's own type and colour system. `.icon.on` / `.icon.danger` / `.primary` keep driving colour. No icon font or library added.

## No-cover empty state
Series whose source site gave no `og:image` render a title-initial monogram (`Bookmark.Initial()`) instead of a blank `--surface-2` rectangle that read as a cover still loading. Shared between the card cover and the Continue-reading strip.

## Verification
- `go build ./... && go test ./...` — pass
- Live interaction run: favourite toggle round-trips, chapter save 210.5→211, All/Updated/Favourites swap, delete-confirm removes the card, no console errors

Reviewed-on: #3
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/templates/app.html

@@ -25,6 +25,9 @@
     <a role="tab" href="/?tab=all" class="{{if eq .Tab "all"}}active{{end}}"
        hx-get="/ui/list?tab=all" hx-target="#list" hx-swap="innerHTML"
        hx-push-url="/?tab=all" hx-on::after-request="setActiveTab(this)">All</a>
+    <a role="tab" href="/?tab=new" class="tab-new {{if eq .Tab "new"}}active{{end}}"
+       hx-get="/ui/list?tab=new" hx-target="#list" hx-swap="innerHTML"
+       hx-push-url="/?tab=new" hx-on::after-request="setActiveTab(this)">Updated</a>
     <a role="tab" href="/?tab=fav" class="{{if eq .Tab "fav"}}active{{end}}"
        hx-get="/ui/list?tab=fav" hx-target="#list" hx-swap="innerHTML"
        hx-push-url="/?tab=fav" hx-on::after-request="setActiveTab(this)">Favourites</a>
@@ -36,7 +39,11 @@
     <div class="recent-strip">
       {{range .Recent}}
       <a class="recent-card" href="{{.ContinueURL}}" target="_blank" rel="noopener noreferrer">
-        {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy">{{end}}
+        <span class="recent-cover">
+          {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy">
+          {{else}}<span class="cover-blank" aria-hidden="true">{{.Initial}}</span>{{end}}
+          {{if .HasNewChapter}}<span class="new-ribbon new-ribbon-sm">New</span>{{end}}
+        </span>
         <span class="recent-title">{{.Title}}</span>
         <span class="recent-chapter">Ch {{.LastChapter}}</span>
       </a>