Add PWA support — installable as home screen app

Adds Web App Manifest, a minimal Service Worker, and Apple/Android meta
tags so the app can be added to a phone home screen and opens full-screen
in standalone mode (no browser chrome).

- static/manifest.json: name, short_name, display=standalone, icons
- static/sw.js: minimal SW served at /sw.js (root scope) via new Flask route
- static/icon-192.png, icon-512.png: generated by sbin/gen_icons.py (stdlib only)
- base.html: manifest link, theme-color, apple-mobile-web-app-* tags, SW registration

sbin/gen_icons.py

@@ -0,0 +1,31 @@
+"""Generate solid-color PNG icons for PWA manifest using stdlib only (no Pillow)."""
+import zlib, struct, os
+
+def make_png(size, rgb=(0x25, 0x63, 0xEB)):
+    """Create a minimal valid RGB PNG of the given size filled with one color."""
+    w = h = size
+    raw = b''
+    for _ in range(h):
+        raw += b'\x00' + bytes(rgb) * w  # filter byte 0 (None) + RGB pixels
+
+    compressed = zlib.compress(raw, 9)
+
+    def chunk(name, data):
+        c = name + data
+        return struct.pack('>I', len(data)) + c + struct.pack('>I', zlib.crc32(c) & 0xffffffff)
+
+    sig  = b'\x89PNG\r\n\x1a\n'
+    ihdr = chunk(b'IHDR', struct.pack('>IIBBBBB', w, h, 8, 2, 0, 0, 0))
+    idat = chunk(b'IDAT', compressed)
+    iend = chunk(b'IEND', b'')
+    return sig + ihdr + idat + iend
+
+
+if __name__ == '__main__':
+    out_dir = os.path.join(os.path.dirname(__file__), '..', 'static')
+    os.makedirs(out_dir, exist_ok=True)
+    for size in [192, 512]:
+        path = os.path.join(out_dir, f'icon-{size}.png')
+        with open(path, 'wb') as f:
+            f.write(make_png(size))
+        print(f'Wrote {path}')