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.go

@@ -35,11 +35,21 @@ type webHandler struct {
 
 // listView is what every list-rendering template receives.
 type listView struct {
-	Tab    string // "all" or "fav"
+	Tab    string // "all", "fav", or "new"
 	Recent []Bookmark
 	Items  []Bookmark
 }
 
+// Initial is the monogram the templates show in place of a cover when the
+// source site never gave us an og:image. First rune, uppercased; "?" when even
+// the title is missing, so the slot is never empty.
+func (b Bookmark) Initial() string {
+	for _, r := range b.Title {
+		return strings.ToUpper(string(r))
+	}
+	return "?"
+}
+
 // loginView is what the login template receives.
 type loginView struct {
 	Error string
@@ -150,14 +160,22 @@ func (h *webHandler) buildListView(tab string) (listView, error) {
 	}
 
 	items := all
-	if tab == "fav" {
+	switch tab {
+	case "fav":
 		items = []Bookmark{}
 		for _, b := range all {
 			if b.Favorite {
 				items = append(items, b)
 			}
 		}
-	} else {
+	case "new":
+		items = []Bookmark{}
+		for _, b := range all {
+			if b.HasNewChapter() {
+				items = append(items, b)
+			}
+		}
+	default:
 		tab = "all"
 	}
 	return listView{Tab: tab, Recent: recent, Items: items}, nil