feat(webui): minify and gzip-compress the embedded web assets

Two build-time steps shrink the embedded Web UI (html/) from 113,015 to
34,784 bytes (30.8 %):

- tools/minify.py (new): string-literal-safe minifier that strips
  comments and indentation; comments are pure overhead on the wire, so
  the transfer size drops already before gzip (113,015 -> 101,498).
  Binary files (.ico) are copied through unchanged.  The output goes to
  output/html_min/, the new fileadder input.
- fileadder -z: gzip-compresses every added file (zlib gzip format,
  Z_BEST_COMPRESSION); the generated f_data entries carry a gzip flag.
  tools/Makefile links the tool against -lz.
- httpd: adds "Content-Encoding: gzip" for flagged entries; the 8051
  only copies compressed bytes, decompression happens in the browser.
  The JSON API stays uncompressed.

Fixes required to make the sizes deterministic:
- fileadder terminated files at data_read + 1, so the strlen()-based
  size computation depended on uninitialised buffer content; the NUL
  now lands directly after the content.
- version.h was .PHONY, so every build rewrote it while parallel builds
  could read a half-written copy.
- the file list was unscoped and missed .css/.ico; it is now scoped to
  html/ and covers all embedded types.

Measured with make MACHINE=KP_9000_6XHML_X2: data block end moves from
0x5b977 to 0x487e0 (~78 KB freed on a 512 KB image).  Verified with
MACHINE=PCB_K0402WS_V3 and on real hardware (PCB-K0402WS-V3).

tools/minify.py

@@ -0,0 +1,72 @@
+#!/usr/bin/env python3
+"""Safe minifier for the WebUI sources (HTML/JS).
+
+Removes comments and line-leading whitespace without touching string
+literals, so the output is functionally identical to the input.  Used
+by the firmware Makefile before the files are embedded in the flash;
+the raw sources in html/ stay untouched for development.
+
+Binary files (.ico) are copied through unchanged.
+
+Usage: minify.py <src> <dst>
+"""
+import re
+import shutil
+import sys
+
+
+def js_min(s):
+    out = []
+    i = 0
+    n = len(s)
+    while i < n:
+        c = s[i]
+        if c == '"' or c == "'":
+            j = i + 1
+            while j < n and s[j] != c:
+                if s[j] == '\\':
+                    j += 1
+                j += 1
+            out.append(s[i:j + 1])
+            i = j + 1
+        elif c == '/' and i + 1 < n and s[i + 1] == '/':
+            j = s.find('\n', i)
+            i = j if j >= 0 else n
+        elif c == '/' and i + 1 < n and s[i + 1] == '*':
+            j = s.find('*/', i + 2)
+            i = j + 2 if j >= 0 else n
+        else:
+            out.append(c)
+            i += 1
+    lines = [l.strip() for l in ''.join(out).split('\n')]
+    return '\n'.join(l for l in lines if l) + '\n'
+
+
+def html_min(s):
+    s = re.sub(r'<!--.*?-->', '', s, flags=re.S)
+    lines = [l.strip() for l in s.split('\n')]
+    return '\n'.join(l for l in lines if l) + '\n'
+
+
+def main():
+    if len(sys.argv) != 3:
+        print('usage: minify.py <src> <dst>', file=sys.stderr)
+        sys.exit(1)
+    src, dst = sys.argv[1], sys.argv[2]
+    if src.endswith('.ico'):
+        shutil.copyfile(src, dst)
+        print('%s: copied %d bytes' % (src.split('/')[-1], len(open(src, 'rb').read())))
+        return
+    with open(src, 'r', encoding='utf-8', errors='replace') as f:
+        s = f.read()
+    if src.endswith('.js'):
+        out = js_min(s)
+    else:
+        out = html_min(s)
+    with open(dst, 'w', encoding='utf-8') as f:
+        f.write(out)
+    print('%s: %d -> %d bytes' % (src.split('/')[-1], len(s), len(out)))
+
+
+if __name__ == '__main__':
+    main()