feat: let subscriptions be renamed from the list (#1044)

A subscription's name is captured once at subscribe time from the feed's own
title, so playlists — particularly the UULF-prefixed channel-uploads playlists
— all come back named "Videos" and stay that way. Adds an inline editor on the
Name cell, mirroring the existing title-filter edit next to it.

The update route already whitelisted `name` and update_subscription already
applied it, so this is mostly the missing UI. The backend side is validation:
the old `str(changes["name"])` accepted any type, any length and any
whitespace, for a value that is persisted and broadcast to every connected
client. validate_subscription_name now requires a string, collapses interior
whitespace to keep the label single-line, and caps it at 200 characters.

The name is display-only — it is used for the subscription list and log lines,
never for download paths — so renaming cannot move where files land.

ui/src/app/app.html

@@ -958,7 +958,33 @@
                 [disabled]="downloads.loading"
                 [attr.aria-label]="'Select subscription ' + entry[1].name" />
             </td>
-            <td>{{ entry[1].name }}</td>
+            <td>
+              @if (editingNameId === entry[0]) {
+                <div class="d-flex flex-wrap gap-1 align-items-center">
+                  <input type="text"
+                    class="form-control form-control-sm flex-grow-1"
+                    [name]="'subName' + entry[0]"
+                    [(ngModel)]="nameEditDraft"
+                    [maxlength]="subscriptionNameMaxLength"
+                    [disabled]="downloads.loading"
+                    [attr.aria-label]="'Subscription name for ' + entry[1].name" />
+                  <button type="button" class="btn btn-sm btn-outline-secondary"
+                    (click)="saveName(entry[0])"
+                    [disabled]="downloads.loading">Save</button>
+                  <button type="button" class="btn btn-sm btn-outline-secondary"
+                    (click)="cancelEditName()"
+                    [disabled]="downloads.loading">Cancel</button>
+                </div>
+              } @else {
+                <div class="d-flex flex-wrap gap-1 align-items-center">
+                  <span class="text-break">{{ entry[1].name }}</span>
+                  <button type="button" class="btn btn-link btn-sm p-0"
+                    (click)="beginEditName(entry[0], entry[1].name)"
+                    [disabled]="downloads.loading"
+                    ngbTooltip="Rename this subscription (display name only; does not affect the download folder)">Edit</button>
+                </div>
+              }
+            </td>
             <td class="text-break"><a [href]="entry[1].url" target="_blank" rel="noopener">{{ entry[1].url }}</a></td>
             <td>
               @if (editingTitleRegexId === entry[0]) {