fix: apply Heading style from BASE_STYLE_MAP level in Word output

[Shengshenlan]

Problem

BASE_STYLE_MAP in word_converter.py already defines a level field for title-type blocks:

BASE_STYLE_MAP = {
    "doc_title":       {"level": 0, "size": 20, "bold": True, "align": CENTER},
    "content_title":   {"level": 1, "size": 14, "bold": True, "align": LEFT},
    "reference_title": {"level": 1, "size": 14, "bold": True, "align": LEFT},
    "paragraph_title": {"level": 2, "size": 14, "bold": True, "align": LEFT},
    ...
}

However, _set_paragraph_style() never reads level — it only applies size, bold, align, and indent. As a result, all paragraphs in the generated .docx are Normal style, including detected titles. Word's navigation pane, outline view, and auto-generated TOC are all empty.

The level field is defined but never consumed — this looks like a half-finished feature.

Fix

Wire up the level field in _set_paragraph_style:

• Added a doc parameter (defaults to None, so existing callers are unaffected)
• When config["level"] is present and doc is available, assign the corresponding Word style:
  • level == 0 → Title
  • level == N → Heading N
• Updated the 3 call sites in _write_block to pass doc

Before / After

Tested on a real 3-page tender document (中文招标文件):

Before (all Normal):

Normal: 11   ← no headings at all

After (headings applied):

第1页: Title: 1    ← "电子化货物采购招标文件"
第2页: Heading 2: 6  ← "第一章招标公告", "1、招标条件", "2、项目基本情况"...
第3页: Heading 2: 2  ← "3.1、投标人应依法设立...", "3.2、本次招标不接受联合体"

Word navigation pane now shows the heading tree, and auto-TOC works.

Backward compatibility

• doc parameter defaults to None — if not passed, behavior is unchanged (same as before)
• Style assignment is wrapped in try/except — never breaks if a style is missing

Resolves #4207

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