Add YouTube Shorts tab (C# port of amigadb-short.sh)

New "Video | Shorts" tab strip on the main page. The existing Video tab and
its render pipeline are unchanged; Shorts is a new, independent workflow
sharing the same render-progress/log/output controls.

- Services/ShortsPipeline.cs: C# port of amigadb-short.sh rather than
  shelling out to bash — end users on plain Windows don't have bash/WSL, and
  shipping this as a native part of the app keeps distribution self-contained.
  All 19 visual style filtergraphs (brand, pixel, mirror, crop, workbench,
  blur, crt, copper, stars, grid, tiles, scan, starfield, plasma,
  rasterbars, vhs, monitor, split, spectrum) are copied verbatim from the
  bash heredocs. Per explicit decision, kept parity with the script's actual
  behavior where the brand-style HOOK_BOX_Y/INFO_BOX_Y/font-size variables
  are computed but never actually used in the final filter (dead code in
  the original) rather than "fixing" it and changing rendered output.
  Reuses ToolExtractor/MediaProbe/ProcessRunner exactly as RenderPipeline
  does, so FFmpeg resolution and NVENC fallback behave identically.

- Models/ShortSettings.cs: job parameters + ShortStyle enum.

- AppSettings gains sticky Shorts defaults (style, hook, website, font,
  background image, CRF, preset) alongside the existing Video defaults.

- MainPage: Video/Shorts tab buttons toggle two content panels; the
  background-image field auto-hides for non-brand styles; font field
  defaults to the first bold system font found (Segoe UI Bold, Segoe UI
  Semibold, Arial Bold, Calibri Bold); blank output path is auto-derived
  from the input filename and style, matching the script's default.

- 13 new unit tests (escaping order, meta-line joining, validation,
  every style producing a non-empty filter, the drawtext trailer's timing
  gates). Full run is 72/72 passing.

- Fixed a bug caught during manual testing: MainPage.Resources["PrimaryButtonStyle"]
  looked in the Page's own (empty) resource dictionary instead of
  Application.Current.Resources, throwing on every tab switch.

Verified end-to-end: rendered a real Short from an existing video through
the actual running app (not just unit tests) and inspected extracted
frames — hook/title/meta/website text overlays appear and disappear at the
correct timestamps with correct styling.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Models/ShortSettings.cs

@@ -0,0 +1,47 @@
+namespace AmiReel.Models;
+
+/// <summary>Visual style applied to a YouTube Short. Mirrors the --style values of amigadb-short.sh.</summary>
+public enum ShortStyle
+{
+    Brand,
+    Pixel,
+    Mirror,
+    Crop,
+    Workbench,
+    Blur,
+    Crt,
+    Copper,
+    Stars,
+    Grid,
+    Tiles,
+    Scan,
+    Starfield,
+    Plasma,
+    Rasterbars,
+    Vhs,
+    Monitor,
+    Split,
+    Spectrum,
+}
+
+/// <summary>One YouTube Shorts render job's parameters (port of amigadb-short.sh's arguments).</summary>
+public sealed record ShortSettings
+{
+    public required string InputFile { get; init; }
+    public required string OutputPath { get; init; }
+    public required string Title { get; init; }
+    public string Group { get; init; } = "";
+    public string Year { get; init; } = "";
+    public string Type { get; init; } = "";
+    public required string Hook { get; init; }
+    public required string Website { get; init; }
+    public required string Start { get; init; }
+    public required double DurationSeconds { get; init; }
+    public required ShortStyle Style { get; init; }
+    public string BackgroundImage { get; init; } = "";
+    public required string FontFile { get; init; }
+    public required string FfmpegPath { get; init; }
+    public required string FfprobePath { get; init; }
+    public int Crf { get; init; } = 16;
+    public string Preset { get; init; } = "slow";
+}