Fix quick fixes / known bugs from TODO section 1

[aaralh]

Fixes all 11 items in TODO.md section 1 (quick fixes / known bugs), one commit per bug, with unit tests for everything testable headlessly.

Engine (web/)

• Percentage font-size with a parent resolved to nothing — the computation was misindented into the no-parent branch of computed_style, so the property was silently dropped for any element with a parent.
• raise NotImplemented → raise NotImplementedError in tag_is_special (the constant is not an exception, so it raised TypeError).
• EventInit trailing commas made bubbles/cancelable default to the truthy tuple (False,) instead of False.
• DomException now subclasses Exception (was BaseException), carries its message through super().__init__, and is actually raised: Node.removeChild/appendChildBeforeElement raise it with name NotFoundError per the DOM spec instead of leaking a bare ValueError.

Application (browser/)

• rem/em width/height math — the width branches assigned a str to font_size and then multiplied it, string-repeating into self.width and crashing later int() calls; the height branches crashed on any non-px font-size. Both now use the already-resolved self.font_size. (rem still uses the element font size as its base, matching create_margin/create_padding; the proper root-based distinction is tracked in TODO §3.)
• Keyword font sizes — convert_absolute_size_to_pixels returned 16 for everything; xx-small…xxx-large now map to the standard scale on a 16px medium base.
• Scrolling used window width for height — scroll_down's max-scroll plus the scrollbar thumb updates in scroll_down/scroll_up/scrollbar_scroll all read get_window_size()[0], letting pages scroll past their end.
• internal_padding clobbering — every create_border branch overwrote it (last write won), so e.g. a lone border-top-width horizontally indented text. It's now derived once from the left border width, matching its sole use as TextLayout's x inset.
• DrawBorder.calculate_offset body was commented out (always (0, 0)); restored the corner-mitre logic so mixed-width borders meet at corners.
• Debug artifacts — raster() no longer writes rules.txt/document.html to the CWD on every load; removed stray prints in ImageLayout and transform_color.
• Swallowed load errors — check_key's bare except: pass now catches Exception and logs the failure.

Tests & typing

• 29 new unit tests across 6 new files (test_computed_style, test_parser_utils, test_event, test_dom_exception, test_font_utils, test_layout_units); suite grows 47 → 76, all green via ./test.sh.
• Zero new mypy errors; 66 pre-existing ones eliminated (mypy had been flagging three of these bugs — the missing return and the tuple defaults — all along).

Follow-ups (out of scope here)

• internal_padding still ignores left padding; padding handling is separate.
• Proper rem (root font size) vs em distinction — TODO §3.

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