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/web_test.go

@@ -483,3 +483,33 @@ func TestUIListFavouritesTab(t *testing.T) {
 		t.Fatal("favourites tab included a non-favourite")
 	}
 }
+
+func TestUIListNewTab(t *testing.T) {
+	cfg := webConfig()
+	srv, store := newWebTestServer(t, cfg)
+	seed(t, store, Bookmark{
+		Key: "asura:solo", Site: "asura", SeriesID: "solo",
+		Title: "Solo Leveling", LastChapterNum: 10,
+		LatestChapter: "Chapter 12", LatestChapterNum: floatPtr(12),
+		UpdatedAt: 2_000_000,
+	})
+	seed(t, store, Bookmark{
+		Key: "demonic:tower", Site: "demonic", SeriesID: "tower",
+		Title: "Tower of God", LastChapterNum: 5,
+		LatestChapter: "Chapter 5", LatestChapterNum: floatPtr(5),
+		UpdatedAt: 1_000_000,
+	})
+
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, uiRequest(t, cfg, http.MethodGet, "/ui/list?tab=new", nil))
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	body := rr.Body.String()
+	if !strings.Contains(body, "Solo Leveling") {
+		t.Fatal("new tab omitted the series with an unread chapter")
+	}
+	if strings.Contains(body, "Tower of God") {
+		t.Fatal("new tab included a series already caught up")
+	}
+}