feat(webapp): +/- sign toggle on weight input (feedback #9)

iOS numeric keypad has no minus key, so users couldn't enter
negative weights for assisted bodyweight exercises (e.g. -20 kg
assisted pull-ups).

Added a small +/- button next to the weight input that flips the
sign of whatever's there (or seeds a lone "-" when the field is
empty so the user can type digits after it). Active state lights
up the button so you can see at a glance that the value is
negative. Parses and stores as a negative REAL in SQLite; existing
display code (fmtWeight) already handles negative numbers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

webapp/index.html

@@ -42,6 +42,7 @@
           <input type="text" id="inp-reps" class="input input-small" placeholder="Reps" inputmode="numeric" pattern="[0-9]*" />
           <span class="set-separator">x</span>
           <input type="text" id="inp-weight" class="input input-small" placeholder="kg" inputmode="decimal" />
+          <button id="btn-weight-sign" class="btn-sign" title="Flip sign (for assisted exercises)" type="button">&plusmn;</button>
           <button id="btn-add-set" class="btn-icon" title="Add set">+</button>
         </div>
       </div>